1. Field of the Invention
The present disclosure relates to control of processes for making acetic acid by carbonylation of methanol or carbonylatable derivative thereof, and particularly to process control during process upsets and during recovery therefrom.
2. Technical Background
Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329 issued to Paulik et al. on Oct. 30, 1973. The carbonylation catalyst contains rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter such as methyl iodide. The rhodium can be introduced into the reaction system in any of many forms, and the exact nature of the rhodium moiety within the active catalyst complex is uncertain. Likewise, the nature of the halide promoter is not critical. The patentees disclose a very large number of suitable promoters, most of which are organic iodides. Most typically and usefully, the reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved or suspended.
An improvement in the prior art process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in commonly assigned U.S. Pat. No. 5,001,259, issued Mar. 19, 1991; U.S. Pat. No. 5,026,908, issued Jun. 25, 1991; and U.S. Pat. No. 5,144,068, issued Sep. 1, 1992; and European Patent No. EP 0 161 874 B2, published Jul. 1, 1992. As disclosed therein, acetic acid is produced from methanol, or a carbonylatable derivative thereof, in a reaction medium containing methyl acetate, methyl halide, especially methyl iodide, and rhodium present in a catalytically effective concentration. These patents disclose that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low water concentrations, i.e. 4 weight percent or less, in the reaction medium (despite the general industrial practice of maintaining approximately 14-15 wt % water) by maintaining in the reaction medium, along with a catalytically effective amount of rhodium and at least a finite concentration of water, a specified concentration of iodide ions over and above the iodide content which is present as methyl iodide or other organic iodide. The iodide ion is present as a simple salt, with lithium iodide being preferred. The patents teach that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid, especially at low reactor water concentrations. By using relatively high concentrations of the methyl acetate and iodide salt, one obtains a surprising degree of catalyst stability and reactor productivity even when the liquid reaction medium contains water in concentrations as low as about 0.1 wt %, so low that it can broadly be defined simply as “a finite concentration” of water. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e. resistance to catalyst precipitation, especially during the product recovery steps of the process. In these steps, distillation for the purpose of recovering the acetic acid product tends to remove from the catalyst the carbon monoxide which in the environment maintained in the reaction vessel, is a ligand with stabilizing effect on the rhodium. U.S. Pat. Nos. 5,001,259, 5,026,908 and 5,144,068 are incorporated herein by reference.
As with any complex chemical process, the methanol carbonylation process described above requires monitoring and control of a number of process conditions such as methanol and carbon monoxide feed rates, reactor temperature and pressure, flasher temperature and pressure, distillation conditions, and the like. In particular, process conditions are carefully controlled to ensure that the acetic acid product is extremely pure, and in particular that it is substantially free of water, methanol and propionic acid. Consequently, when one or more of these process conditions changes suddenly due to an unexpected event such as a sudden decrease in carbon monoxide supply, failure of a catalyst pump, or the like, the production rate must be adjusted—usually downward—to ensure that the acetic acid product continues to meet quality specifications. It is desirable, however, to return to normal operating conditions as rapidly as possible after a process disturbance. It has been observed, however, that process controllers employing standard linear control algorithms do not provide sufficiently rapid recovery from large-magnitude process disturbances because the controllers are tuned to maintain control over the narrow range of “normal” operating conditions rather than the broad range resulting from a significant disturbance. In particular, linear controllers are limited in that the controller gain (i.e., the relationship between the magnitude of a deviation from target conditions associated with certain control variable(s), and the magnitude of the corrective control action achieved using manipulated variable(s)) is fixed rather than changeable. An example of a gain is the amount of steam flow change required to a heat exchanger to cause a one degree change in temperature of a process stream. During rate changes, such as upsets, the composition of the process stream will change resulting in a change in the amount of steam required to affect the one degree change in temperature. Because of this limitation of linear controllers, most multivariable predictive controllers are not capable of maintaining control and recovering quickly from large-magnitude process disturbances. Even where these controllers operate based on an empirical or theoretical model of the process, an underlying assumption of their control scheme is usually that the process gains (i.e., the magnitude of the process's response to a control action) are more or less linear. This assumption turns out to be somewhat unreliable for chemical processes, particularly where the deviation from the target conditions is very large or where a number of interrelated reactions are occurring simultaneously. This is exactly the situation in an acetic acid reactor, where in addition to methanol carbonylation, one methanol Is molecule can react (reversibly) with an acetic acid molecule to form methyl acetate and water; two methanol molecules can react to form dimethyl ether and water; and the methyl acetate can also react directly with carbon monoxide and water to form acetic acid. In fact, it turns out that at least some of the process gains for a methanol carbonylation reactor are not only nonlinear but actually change sign depending on the process conditions. During significant process upsets in a methanol carbonylation process, gains are particularly unlikely to be constant, making linear control less effective.
Notwithstanding the perceived deficiencies of linear model-based controllers for acetic acid reaction systems, it has generally not been considered appropriate to employ nonlinear controllers for this application. Until now, it has generally been thought that nonlinear controllers are best employed in environments where process setpoints are deliberately changed (e.g. to change a product grade) and the objective is to minimize the transition time between grades. Existing nonlinear control applications have focused on production of polymers where there are frequent changes in product grades. These applications have not focused on rate related changes. There remains a need, however, for control systems that are capable of managing nonlinear processes in response to unexpected disturbances so as to provide rapid recovery.
One such system now available commercially is a system from ASPEN Technology that employs two separate components to manage a process upset. In addition to a dynamic controller for maintaining control until a disruption has been addressed, ASPEN's solution employs a separate gain-scheduling component that is designed to manage the return to normal operating conditions. In effect, the gain scheduler treats the return from abnormal to normal conditions as a grade change and imposes upon the return a series of essentially linear transitions. Nevertheless, there remains a need for a control system that integrates these components. The present disclosure achieves this objective.